Comcast (NSDQ: CMCSA) Interactive Capital, the venture arm of Comcast, has invested an undisclosed amount in a seed round of Cartiza Networks, a Boston-based stealth WiMax startup…the seed round was a big $12.6 million, and investors include Comcast Interactive Capital, Prism VentureWorks and the Business Development Bank of Canada (BDC), according to this story. Not sure what Cartiza exactly offers, but its website says this: it is developing an Internet Protocol-based “mobile content delivery system” designed to provide “mobile broadband networking, scalable bandwidth and highly sophisticated content management capabilities for voice, data and video.”

Comcast is also involved in the big Clearwire-Sprint (NYSE: S) JV, and is investment about $1.05 billion of its money into it…that venture also includes Time Warner (NYSE: TWX) Cable, Bright House Networks, Intel (NSDQ: INTC) and Google.